Nonvolatile memory devices

ABSTRACT

A nonvolatile memory device includes a semiconductor substrate, a device isolation film, a tunnel insulation film, a plurality of floating gates, an inter-gate dielectric film, and a control gate pattern. Trenches are formed in the substrate that define active regions therebetween. The device isolation film is in the trenches in the substrate. The tunnel insulation film is on the active regions of the substrate. The plurality of floating gates are each on the tunnel insulation film over the active regions of the substrate. The inter-gate dielectric film extends across the floating gates and the device isolation film. The control gate pattern is on the inter-gate dielectric film and extends across the floating gates. A central region of the device isolation film in the trenches has an upper major surface that is recessed below an upper major surface of a surrounding region of the device isolation film in the trenches. An edge of the recessed central region of the device isolation film is aligned with a sidewall of an adjacent one of the floating gates.

CROSS-REFERENCE TO RELATED APPLICATIONS

This U.S. non-provisional patent application claims priority under 35U.S.C. § 119 to Korean Patent Application 10-2004-0061472 filed on Aug.4, 2004, the entire contents of which are hereby incorporated byreference.

FIELD OF THE INVENTION

The present invention relates to nonvolatile memory device technologyand, more particularly, to floating gate non-volatile memory devices andmethods of fabricating the same.

BACKGROUND OF THE INVENTION

The integration density and storage capacity of nonvolatile memorydevices have been increased by decreasing the size of active regions andisolation films within cell array regions of the devices. One approachfor fabricating such devices uses a self-aligned polysilicon fabricationprocess. The circuit area of a memory device may be reduced using theself-aligned polysilicon fabrication process by forming a floating gatepattern on an area that is confined between adjacent device isolationfilms.

U.S. Pat. No. 6,656,793, entitled “Method of forming a self alignedfloating gate in flash memory cell”, discloses a process for enlarging asurface area of a floating gate pattern through a self-alignedpolysilicon fabrication process. After forming the floating gate patternon a region that is defined between adjacent device isolation film, thedevice isolation film is partially removed through an isotropic etchingprocess to expose sidewalls of the floating gate pattern.

FIG. 1 is a cross sectional view of a conventional nonvolatile memorydevice. Referring to FIG. 1, a semiconductor substrate 50 is grooved toform trenches with active regions 59 therebetween, and a deviceisolation film 58 is formed in the trenches. A tunnel insulation film(60 in FIGS. 3 and 4) is formed on the active regions 59 of thesubstrate 50. Floating gates 62 are formed on the tunnel insulation film(60 in FIGS. 3 and 4) over the active regions 59 of the substrate 50.

An inter-gate dielectric film 64 is formed on the floating gates 62 andthe device isolation film. A control gate pattern 66 is deposited on theinter-gate dielectric film 64 over the floating gates 62. An uppersurface of the device isolation film 58 is lower than an upper surfaceof the floating gates 62, and the interlevel dielectric film 64 covers aportion of sidewalls of the floating gates 62. The floating gates 62 arewider than the substrate 50 between the trenches, so that they partiallyoverlap the trenches. Forming the control gate pattern 66 on sidewallsof the floating gates 62 and overlapping the floating gates 62 with aportion of the trenches can increase the capacitive coupling between thecontrol gate pattern 66 and the floating gates 62.

FIGS. 2, 3, and 4 are cross sectional views of conventional nonvolatilememory devices and illustrate methods of fabricating the same.

Referring to FIG. 2, the substrate 50 is etched to form the plurality oftrenches. The device isolation film 58 is deposited to fill thetrenches. After its deposition, the device isolation film 58 includesprojections that extend away from the substrate 50.

Referring to FIG. 3, the device isolation film 58 is recessed by anisotropic etching process so that its upper surface is below an uppersurface of the substrate 50, leaving the tunnel insulation film 60between the floating gates 62 and the substrate 50. Each of the floatinggates 62 is wider than the width of the active region 59 of thesubstrate 50 between the trenches. The isotropic etching of the deviceisolation films 58 at least partially exposes sidewalls of the floatinggates 62.

When the sidewalls of the floating gates 62 are completely exposed byetching of the device isolation film 58, the tunnel insulation film 60between the floating gates 62 and the substrate 50 may be partiallyremoved. Such partial removal of the tunnel insulation film 60 maydegrade the uniformity of the operational characteristics of a cellarray in the memory device that includes such structures. Partialremoval of the tunnel insulation film 60 may be avoided by onlypartially exposing the sidewalls of the floating gates 62 when etchingthe device isolation film 58. However, only partially exposing thesidewalls of the floating gates 62 may not provide an adequatecapacitive coupling between the floating gates 62 and a subsequentlyformed control gate pattern 66.

Referring to FIG. 4, the inter-gate dielectric film 64 is conformablyformed over the entire exposed upper surface of the structure of FIG. 3.The control gate film 66 is deposited on the inter-gate dielectric film64. As shown in FIG. 4, opposite edge portions of a lower surface of thefloating gates 62 are directly on the inter-gate dielectric film 64, andthe control gate film 66 extends within an undercut region 70 below theopposite edge portions of the floating gates 62. The control gate film66 within the undercut region 70 may form an undesirable conductivebridge between some of the floating gates 62.

SUMMARY OF THE INVENTION

Some embodiments of the present invention provide a nonvolatile memorydevice. Trenches are formed in a semiconductor substrate that defineactive regions therebetween. A device isolation film is in the trenchesin the substrate. A tunnel insulation film is on the active regions ofthe substrate. A plurality of floating gates are each on the tunnelinsulation film over the active regions of the substrate. An inter-gatedielectric film extends across the floating gates and the deviceisolation film. A control gate pattern is on the inter-gate dielectricfilm and extends across the floating gates. A recessed region of thedevice isolation film in the trenches has an upper major surface that isrecessed below an upper major surface of a surrounding region of thedevice isolation film in the trenches. An edge of the recessed region ofthe device isolation film is aligned with a sidewall of an adjacent oneof the floating gates.

Some other embodiments of the present invention provide a method offabricating a nonvolatile memory device. Trenches are formed in asemiconductor substrate that define active regions therebetween. Adevice isolation film is formed that fills the trenches in thesubstrate. A tunnel insulating film is formed on the active regions ofthe substrate. A plurality of floating gates are formed with each of thefloating gates on the tunnel insulating film over the active regions ofthe substrate. A portion of the device isolation film is removed overthe trenches in the substrate to expose sidewalls of the floating gates.A recessed region of the device isolation film in the trenches isrecessed so that the recessed region has an upper major surface that isrecessed below an upper major surface of a surrounding region of thedevice isolation film in the trenches and an edge of the recessed regionof the device isolation film is aligned with a sidewall of an adjacentone of the floating gates. An inter-gate dielectric film is formed thatextends across the floating gates and the device isolation film. Acontrol gate conductive film is formed on the inter-gate dielectric filmand extending across the floating gates.

Some other embodiments of the present invention provide a method offabricating a nonvolatile memory device. Trenches are formed in asemiconductor substrate that define active regions therebetween. Adevice isolation film is formed that fills the trenches in thesubstrate. A tunnel insulating film is formed on the active regions ofthe substrate. A plurality of floating gates are formed with each offloating gates on the tunnel insulating film over the active regions ofthe substrate. A portion of the device isolation film between thefloating gates is isotropically etched to partially expose sidewalls ofthe floating gates. Another portion of the device isolation film betweenthe floating gates is anisotropically etched to completely expose thesidewalls of the floating gates. An inter-gate dielectric film is formedthat extends across the floating gates and the device isolation film. Acontrol gate conductive film is formed on the inter-gate dielectric filmand extending across the floating gates.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate example embodimentsof the present invention and, together with the description, serve toexplain principles of the present invention. In the drawings:

FIG. 1 is a cross sectional view of a conventional nonvolatile memorydevice;

FIGS. 2, 3, and 4 are cross sectional views of conventional nonvolatilememory devices and illustrate methods of fabricating the same; and

FIGS. 5 through 10 are cross sectional views of nonvolatile memorydevices and illustrate methods of fabricating the same in accordancewith various embodiments of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will be described in detail herein. Itshould be understood, however, that there is no intent to limit theinvention to the particular forms disclosed, but on the contrary, theinvention is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims. Like reference numbers signify like elements throughout thedescription of the figures.

It will be understood that when an element or layer is referred to asbeing “on,” “connected to,” or “coupled to” another element or layer, itcan be directly on, connected, or coupled to the other element or layeror intervening elements or layers may be present. In contrast, when anelement is referred to as being “directly on,” “directly connected to,”or “directly coupled to” another element or layer, there are nointervening elements or layers present. As used herein, the term“and/or” includes any and all combinations of one or more of theassociated listed items.

It will be understood that although the terms first and second are usedherein to describe various regions, layers and/or sections, theseregions, layers and/or sections should not be limited by these terms.These terms are only used to distinguish one region, layer or sectionfrom another region, layer or section. Thus, a first region, layer orsection discussed below could be termed a second region, layer orsection, and similarly, a second region, layer or section may be termeda first region, layer or section without departing from the teachings ofthe present invention.

Furthermore, relative terms, such as “lower” or “bottom” and “upper” or“top” may be used herein to describe one element's relationship toanother elements as illustrated in the figures. It will be understoodthat relative terms are intended to encompass different orientations ofthe device in addition to the orientation depicted in the figures. Forexample, if the device in the figures is turned over, elements describedas being on the “lower” side of other elements would then be oriented on“upper” sides of the other elements. The exemplary term “lower” can,therefore, encompass both an orientation of “lower” and “upper,”depending of the particular orientation of the figure. Similarly, if thedevice in one of the figures is turned over, elements described as“below” or “beneath” other elements would then be oriented “above” theother elements. The exemplary terms “below” or “beneath” can, therefore,encompass both an orientation of above and below.

Embodiments of the present invention are described herein with referenceto cross-section illustrations that are schematic illustrations ofidealized embodiments of the present invention. As such, variations fromthe shapes of the illustrations as a result, for example, ofmanufacturing techniques and/or tolerances, are to be expected. Thus,embodiments of the present invention should not be construed as limitedto the particular shapes of regions illustrated herein but are toinclude deviations in shapes that result, for example, frommanufacturing. For example, an implanted region illustrated as arectangle will, typically, have rounded or curved features and/or agradient of implant concentration at its edges rather than a binarychange from an implanted to a non-implanted region. Likewise, a buriedregion formed by implantation may result in some implantation in theregion between the buried region and the surface through which theimplantation takes place. Thus, the regions illustrated in the figuresare schematic in nature and their shapes are not intended to illustratethe precise shape of a region of a device and are not intended to limitthe scope of the present invention.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andwill not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

FIG. 5 is a cross sectional view of a nonvolatile memory device andillustrates methods of fabricating the same in accordance with someembodiments of the present invention.

Referring to FIG. 5, the nonvolatile memory device includes a substrate10 with trenches 16 that define active regions 10 a between the trenches16. A device isolation film 18 is in the trenches 16 of the substrate10. A tunnel insulating film 20 is on the active regions 10 a of thesubstrate 10. A plurality of floating gates 22 each are on the tunnelinsulating film 20 over the active regions 10 a of the substrate 10. Thefloating gates 22 are each wider than the active regions 10 a so thatsidewalls 22 s of the floating gates 22 extend over a portion of thedevice isolation film 18 in the trenches 16. An inter-gate dielectricfilm 24 extends across the floating gates 22 and the device isolationfilm 18. A control gate pattern 26 is on the inter-gate dielectric film24 and extends across the floating gates 22.

In accordance with some embodiments, a central region 18 r of the deviceisolation film 18 has an upper major surface that is recessed below anupper major surface of a surrounding region of the device isolation film18 in the trenches 16, and an edge 18 s of the recessed central region18 r of the device isolation film 18 is aligned with a sidewall 22 s ofan adjacent one of the floating gates 22. The upper major surface of therecessed central region 18 r may be below an upper major surface of theactive regions 10 a of the substrate 10.

Because the floating gates 22 are wider than the active regions 10 a,the edge 18 s of the recessed central region 18 r is aligned with thesidewall 22 s of the floating gates 22 but is electrically isolated fromthe edge of the active region 10 a by the device isolation film 18.Accordingly, the device isolation film 18 can be immediately between theinter-gate dielectric film 24 and edge portions of the active regions 10a. Consequently, the thickness of the device isolation film 18 along theedge of the active regions 10 a between the control gate film 26 and theactive regions 10 a may be adjusted to be sufficiently thick to providea desired threshold for the associated transistor. The thickness of thedevice isolation film 18 can be adjustable by adjusting the width of thefloating gates 22 relative to the width of the active regions 10 aand/or by adjusting the depth of the upper major surface of the recessedcentral regions 18 r of the device isolation film 18 relative to theupper major surface of the surrounding device isolation film 18.

The upper major surface of the recessed central region 18 r of thedevice isolation film 18 may be aligned with or below the lower surfaceof the floating gates 22 so that the inter-gate dielectric film 24completely covers the sidewalls 22 s of the floating gates 22.Accordingly, the floating gates 22 can be wider than the active regions10 a and the device isolation film 18 can be completely removed from thesidewalls 22 s of the floating gates 22. The inter-gate dielectric film24 may therefore be directly between the control gate film 26 and theupper surface and side surfaces 22 s of the floating gates 22, which canincrease the capacitive coupling therebetween.

FIGS. 6 through 10 are cross sectional views of a nonvolatile memorydevice that illustrate methods of fabricating the same in accordancewith some embodiments of the present invention.

Referring to FIG. 6, a hard mask pattern 14 is formed on thesemiconductor substrate 10. The trenches 16 are etched into thesubstrate 10 using the hard mask pattern 14 as an etch mask. The hardmask pattern 14 may be formed from a silicon-nitride film or a stackedsilicon-nitride and silicon-oxide film. Before forming thesilicon-nitride film on the substrate, a buffer oxide film 12 may beformed to reduce stress on the substrate 10. The trenches 16 are filledwith an insulation film that also covers the substrate. The hard maskpattern 14 is exposed by polishing the insulation film by, for example,a chemical/mechanical polishing (CMP) process to form the deviceisolation film 18 in the trenches 16 between the hard mask patterns 14.

Referring to FIG. 7, the active region 10 a confined by the deviceisolation film 18 is exposed by removing the hard mask pattern 14 andthe buffer oxide film 12. The device isolation film 18 can beisotropically etched to recess sidewalls 18 s of the exposed isolationfilm 18 by a recessed width w1. The device isolation film 18 therebyprojects upward from the trenches 18 beyond the upper surface of theactive regions 10 a.

Referring to FIG. 8, a tunnel insulation film 20 is deposited on theactive region 10 a and a conductive film is deposited to fill the spacesbetween the projections of the device isolation film 18. The conductivefilm is polished by, for example, a CMP process to expose the deviceisolation film 18 and form floating gates 22 over the active regions 10a in the regions between the device isolation film 18. Because sidewallsof the device isolation film 18 have been recessed by a width w1, thewidth of the floating gates 22 is wider than the width of the activeregions 10 a. The floating gates 22 can thereby have a larger surfacearea compared to if they were only as wide as the active regions 10. Theedges of the floating gates 22 extend over a portion of the trenches 16and overlap a portion of the device isolation film 18.

Referring to FIG. 9, the device isolation film 18 is recessed topartially expose sidewalls of the floating gates 22. The deviceisolation film 18 can be isotropically etched to recess it. Byisotropically etching the device isolation film 18 instead ofanisotropic dry etching the device isolation film 18, damage to thefloating gates 22 from ion contamination may be avoided. The tunnelinsulation film 20 between the floating gates 22 and the active regions10 a should not be exposed when the device isolation film 18 is recessedby the above isotropical etching process.

Referring to FIG. 10, the central region 18 r (FIG. 5) of the deviceisolation film 18 in the trenches 16 is then further recessed by ananisotropic etching process that completely exposes the sidewalls 22 sof the floating gates 22. The edge 18 s of the upper major surface ofthe recessed central region 18 r of the device isolation film 18 isaligned with the sidewalls 22 s of the floating gates 22. The edges 18 sof the floating gates 22 thereby overlap a portion of the deviceisolation film 18, so that a portion of the device isolation film 18 isbelow the edges 18 s of the floating gates 22 after the upper majorsurface of the central region 18 r of the device isolation film 18 isrecessed below the bottom surface of the floating gates 22. Accordingly,the edge 18 s of the upper major surface of the recessed central region18 r of the device isolation film 18 is separated from the edge of theactive region 10 a based on the difference between the widths of thefloating gates 22 and the active regions 10 a. The upper surface of therecessed central region 18 r of the device isolation film 18 may berecessed below the upper major surface of the active regions 10 a by adefined depth. Because the central region 18 r of the device isolationfilm 18 is recessed through an anisotropic etching process, damage tothe tunnel oxide film 20 between the floating gates 22 and activeregions 10 a can be avoided. Moreover, when a wet cleaning process iscarried out after the anisotropic etching process, the tunnel insulationfilm 20 can be protected from damage by the device isolation film 18that remains below the edge surfaces 22 s of the floating gates 22.

The inter-gate dielectric film 24 is conformably formed on the resultantstructure. The control gate film 26 is formed on the inter-gatedielectric film 24 extending across the floating gates 22, illustratedby the exemplary structure shown in FIG. 5.

Subsequent processing is then carried out to complete the structure ofthe nonvolatile memory device by well-known semiconductor fabricationtechniques. For instance, the control gate film 26 and inter-gatedielectric film 24 are patterned to form control gate electrodes. Afloating gate electrode can be formed by forming the floating gates 22so they are aligned with and under the control gate electrode. Theportion of the tunnel insulation film 20, which is exposed on thesubstrate 10 after partially removing the floating gates 22, may beremoved by a wet cleaning process or may be used as a buffer layer whensubsequently forming an impurity diffusion layer in the substrate 10.

As discussed above with regard to some embodiments of the presentinvention, the device isolation film 18 can be initially recessed by anisotropic etching process to expose portions of the sidewalls 22 s ofthe floating gates 22, and a central region 18 r can then be recessedbelow a surrounding region of the device isolation film 18 by ananisotropic etching process. When isotropically etching the deviceisolation film 18 to expose portions of the sidewalls 22 s of thefloating gates 22 it can be important to not expose the tunnelinsulation film 20 between the floating gates 22 and the active regions10 a so that it is not damaged by the etching. The sidewalls 22 s of thefloating gates 22 are then completely exposed by anisotropically etchingthe device isolation film 18 to recess the central region 18 r thereof.The amount of overlap of the edges portions of the floating gates 22 andthe device isolation film 18 in the trenches 18 can be adjusted byadjusting the amount of time and/or etching rate of the wet isotropicetching process carried out on the device isolation film 18.

Accordingly, edges of the recessed central region of the deviceisolation film 18 can be aligned with edges of the floating gates 22 byetching the device isolation film 18 using a sequence of isotropicaletching followed by anisotropical etching of the device isolation film18. Therefore, a surface area of the floating gates 22 may be enlargedwhile avoiding damage to the tunnel insulation film 20 between thefloating gates 22 and the active regions 10 a of the substrate 10.Avoiding damage to the tunnel insulation film 20 may improve theuniformity of the characteristics of the memory cells formed thereby inthe memory device, and increasing the surface area of the floating gatesrelative to the active regions of the substrate may increase thecoupling between the control gate and the floating gates of the memorycells.

In concluding the detailed description, it should be noted that manyvariations and modifications can be made to the preferred embodimentswithout substantially departing from the principles of the presentinvention. All such variations and modifications are intended to beincluded herein within the scope of the present invention, as set forthin the following claims.

1. A nonvolatile memory device comprising: a semiconductor substrate;trenches in the substrate that define active regions therebetween; adevice isolation film in the trenches in the substrate; a tunnelinsulation film on the active regions of the substrate; a plurality offloating gates each on the tunnel insulation film over the activeregions of the substrate; an inter-gate dielectric film extending acrossthe floating gates and the device isolation film; and a control gatepattern on the inter-gate dielectric film and extending across thefloating gates, wherein an edge of the recess region of the deviceisolation film in the trench is alianed with a sidewall of an adjacentgate, wherein the floating gate is wider than the active region of thesubstrate, and wherein an edge portion of the floating gate partiallyoverlaps a portion of the device isolation film in the trench.
 2. Thenonvolatile memory device of claim 1, wherein opposite edges of therecessed region of the device isolation film are aligned with facingsidewalls of adjacent ones of the floating gates.
 3. The nonvolatilememory device of claim 1, wherein the upper major surface of therecessed region of the device isolation film is below an upper surfaceof the active region of the substrate.
 4. The nonvolatile memory deviceof claim 1, wherein: opposite edge portions of a lower surface of thefloating gates are directly on the device isolation film.
 5. Thenonvolatile memory device of claim 1, wherein the device isolation filmis immediately between the inter-gate dielectric film and edge portionsof the active regions of the substrate.
 6. The nonvolatile memory deviceof claim 1, wherein the inter-gate dielectric film is directly on atleast one entire sidewall surface of the floating gates.
 7. Anonvolatile memory device comprising: a semiconductor substrate;trenches in the substrate that define active regions therebetween; adevice isolation film in the trenches in the substrate; a tunnelinsulation film on the active regions of the substrate; a plurality offloating gates each on the tunnel insulation film over the activeregions of the substrate; an inter-gate dielectric film extending acrossthe floating gates and the device isolation film; and a control gatepattern on the inter-gate dielectric film and extending across thefloating gates, wherein a recessed region of the device isolation filmin the trenches has an upper major surface that is recessed below anupper major surface of a surrounding region of the device isolation filmin the trenches, and an edge of the recessed region of the deviceisolation film is aligned with a sidewall of an adjacent one of thefloating gates; wherein the recess region has an upper major surfacethat extends laterally across a substantial portion of the trenches inthe substrate and is recessed below an upper major surface of asurrounding region of the device isolation film in the trenches, and thelaterally extending upper maj or surface of the recessed region of thedevice isolation film has an edge that extends parallel to and isaligned with a sidewall of an adjacent floating gate.
 8. The nonvolatilememory device of claim 7, wherein opposite edges of the recessed regionof the device isolation film are aligned with facing sidewalls ofadjacent ones of the floating gates.
 9. The nonvolatile memory device ofclaim 7, wherein the upper major surface of the recessed region of thedevice isolation film is below an upper surface of the active region ofthe substrate.
 10. The nonvolatile memory device of claim 7, wherein:opposite edge portions of a lower surface of the floating gates aredirectly on the device isolation film.
 11. The nonvolatile memory deviceof claim 7, wherein the device isolation film is immediately between theinter-gate dielectric film and edge portions of the active regions ofthe substrate.
 12. The nonvolatile memory device of claim 7, wherein theinter-gate dielectric film is directly on at least one entire sidewallsurface of each of the floating gates.